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Version of the Presentation
Introduction
A century passed under the spell of the Special
Theory of Relativity, but interested general public still
does not have adequate knowledge about it. This five-minute
presentation is for those who would like to understand what
it is all about, but do not think that they would keep their
minds on it for more than five minutes. If you become curious
to learn more, you can find guidelines on how to continue
at the end of the presentation.
The Special Theory of Relativity is connected
with the properties of space and time. Any event is specified
by a place and by a time of happening. In some class of frames
the laws of physics take their simplest form. They are called
inertial frames. If two inertial reference frames with observers
move uniformly relative to each other, there is no way to
tell, which of them is really at rest, because the laws of
physics look the same for these observers.
The Classical Mechanics of Galileo and Newton is based on
this statement, which is called the Principle of Relativity.
Galileo was led to it by the observation that the laws of
mechanics look the same in a ship moving uniformly and in
a ship at rest. Galileo derived how coordinates of some event
in one inertial frame should be connected with its coordinates
in another frame so that the laws of mechanics look the same
in both frames.
Electromagnetic and optical phenomena are described by Maxwell's
equations based on experiment. Originally they were supposed
to be written for a frame "at rest", and it was
not clear whether the laws of electrodynamics look the same
for observers moving uniformly relative to each other, like
the laws of Classical Mechanics do.
According to the naive old understanding it was supposed
that there exists a privileged frame at "absolute rest".
First it was connected with the Earth, then with the Sun,
then with the Galaxy. It was supposed that it is filled with
ether - the medium, in which light is propagating. But experiments
with electromagnetic and optical phenomena were not able to
disclose the absolute motion of the Earth relative to the
ether, and so the Principle of Relativity seemed to be a general
law of Nature.
But Maxwell's equations were changing under the Galileo transformations.
Trying to find transformations under which the laws of electrodynamics
remain the same, Lorentz wrote a new set of transformations
of coordinates and electromagnetic fields. Poincaré
extended them by including also charge densities and currents
emphasizing that they form a group.
In 1905 Einstein investigating electromagnetic phenomena
suggested that they could be described without the notions
"ether" and "absolute rest". He raised
the Principle of Relativity to the status of a postulate.
The consequences based on the Maxwell's Theory of Electrodynamics
appeared to be consistent and the developed theory later came
to be called the Special Theory of Relativity in order to
contrast it with the General Theory of Relativity that was
extending the validity of the Principle of Relativity also
on the gravity.
From Maxwell's equations follows the constancy of the speed
of light for all observers, which means that if an observer
with a source of light is moving with the velocity V relative
to some other observer, the velocity c of light will be the
same for both observers, and not c + V or c - V, as one could
expect in accordance with the Classical Mechanics. The new
formula of addition of velocities gives the correct result.
The validity of the Principle of Relativity for Electrodynamics
led to a change of the concept of space and time. The concept
of a universal, or absolute, time had to be abandoned, and
the Classical Mechanics had to be adjusted to the requirement
of the constancy of the speed of light.
Relativistic effects follow logically from the Theory and
experiment verifies them. They are significant only at high
speeds - mainly for the world of very large and for the world
of very small and seem to be in conflict with our everyday
experience. So called "paradoxes" of the Special
Theory of Relativity are not paradoxical at all, when one
considers them carefully. The key to that lies in correct
determination of place and time of events in different frames.
Time and space can be measured in the same units. It should
be familiar to you from everyday life. People often say: "My
home is ten minutes by car from work." Such statement
implies a speed of the car. Here we shall be implying the
speed of light and shall measure both time and space in meters.
For example, 10 meters of time will mean time in which the
light covers the distance of 10 meters.
Imagine a latticework of sticks and clocks in some frame.
We want them all to read the same time in this frame. In order
to synchronize them, we pick one clock in the lattice. We
set its pointer at zero time and at the same moment send a
flash of light. When the flash of light gets to a clock 5
meters away, we want that clock to read 5 meters of time,
and so on. The space location of an event will be the location
of the clock nearest to the event, and the time of the event
is taken to be the time recorded on the same clock.
Different observers may record different separations in space
and time between two events, but some combination of them,
called spacetime interval, will be the same.
It appears that events simultaneous for one observer might
be not simultaneous for another observer, time is going slower
in moving frames - this way you can, in principle, visit your
descendants, and lengths of moving objects are shorter.
As one of the logical consequences of the Special Theory
of Relativity Einstein found his famous formula
E
= mc²,
expressing the equivalence of mass and energy. How to unlock
the energy hidden inside mass became clear in 1938, when physicists
managed to split an atomic nucleus.
The Principle of Relativity became a general criterion that
all theories must satisfy. The Special Theory of Relativity
initiated an appreciation of the role of symmetry criteria,
which became a powerful guide in the search for the fundamental
laws of Nature. Under the influence of the Special Theory
of Relativity physicists are working towards having as few
laws as possible. There exists even a hope to find a universal
law, the complete theory of all forces - the Theory of Everything.
Nature is diverse and rich in phenomena, but the underlying
laws appear to be beautiful and simple. So the ideas developed
in 1905 are continuing to guide physicists today. The search
for the fundamental laws of Nature continues…
| If you are interested enough and would like to gain
serious knowledge of the subject, you can use the famouse
book of E. F. Taylor and J. A. Wheeler, Spacetime Physics,
W.H Freeman and Company, 1992. |
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